It can be learned, from analysis based on a conventional information theory, that a system capacity is determined by a signal-to-noise ratio of a receive end. Essentially, performing multiple access for a plurality of users is to divide or share the system capacity. In theory, TDMA, CDMA, or (O)FDMA cannot increase a capacity boundary of a system, and are merely different capacity division methods. Emergence of MIMO proves in theory that a system capacity may increase as a quantity of transmit/receive antennas increases, and therefore improves a theoretical boundary of the system. When an LTE system is designed, MIMO and OFDMA are used as basic physical layer technologies of the LTE system to maximize a system capacity. For detection performance improvement of a receiver, a transmitting end usually needs to precode a signal before sending, to reduce interference caused by signals from different antennas to a receiving antenna. In an FDD system, because channel reciprocity is unavailable, a precoding codebook used by a transmitting end needs to be obtained by means of feedback performed by a receive end. In an LTE protocol, UE generally indicates, by feeding back a PMI, an optimal codebook that is used by a BS on the UE.
In the base version (Release 8), LTE supports four transmit antennas on a base station side, and defines codebooks of the four antennas. A UE side needs to perform CSI calculation according to a cell-specific reference signal CRS. Because an antenna quantity is relatively small, and a codebook size is also relatively small, PMI calculation and feedback on the UE side are relatively simple. In LTE R10, a base station side may support a maximum of eight transmit antennas, and a CRS is supported by only four transmit ports, and is supported in each frame. If a CRS is supported by eight antenna ports, pilot overheads are excessively high. Therefore, a CSI-RS that is sent relatively sparsely and may be dynamically configured is introduced to perform CSI measurement and feedback when there are eight antennas. In this case, to reduce PMI calculation complexity on a UE side and a feedback amount, a codebook for the eight antennas uses a double codebook structure, that is, a codebook W of a system includes a first codebook W1 and a second codebook W2, that is, W=W1W2. The first codebook is a wide beam codebook, changes slowly, and has a relatively long UE feedback period. The second codebook is a quickly changing codebook (also referred to as a short-time codebook or a narrow beam codebook). A purpose of the second codebook is to match a frequency selection feature and a short time feature of a channel, and UE needs a relatively short codebook feedback period.
As an antenna quantity of a base station further increases, when a linear array of antennas becomes a two-dimensional planar array of antennas, a configuration of a single CSI-RS cannot support more antenna ports. Theoretically, for a CSI-RS of existing LTE, one resource block (RB) includes a maximum of 40 resource elements (Res). Therefore, 40 orthogonal antenna ports can be supported in theory by constantly increasing a quantity of ports for the CSI-RS. As an antenna quantity continues to increase, a current allocation method in which each antenna port occupies one orthogonal pilot CSI-RS and a method in which UE measures all antenna ports and feeds back an integral PMI are no longer suitable. Therefore, providing a proper channel measurement method under a trend of increasing antenna ports is an urgent problem that needs to be resolved.